strange face appears in edit mode (concave quad)
Short version: Never use concave quads.

Long version: Your topology is not good. I think it can be summed it up this way (the 3D nerds will tell me if I’m wrong):

To create a 3D surface, a software needs 3 vertices, no more (real quads or polygons surfaces would be way too complicated to calculate for a computer), no less (2 vertices make an edge, not a surface, 1 vertex is a point).

So triangles are the basis of 3D. It’s true that we mainly work with quads because it’s much more convenient than triangles but actually the software will always create 2 triangles within the quad to be able to make it a 3D surface.

Now, if you create a ABCD quad, the software has 2 solutions to compose it with 2 triangles: either do it with ABC + ADC or with ABD + BDC. I don’t know how it decides, I guess it’s randomly. Most of the time you won’t notice what way it chose because it won’t make any difference.



But in your case, the triangles choice will indeed make a difference! If it chooses ABC + ADC it will give a correct shape and you won't notice anything, but if it chooses ABD + BDC it won’t give what you want. You would like it to choose the first solution but the software has absolutely no clue that it’s the good way to do it.



So what you need to do is to correct your topology, at least create an edge between A and C.